1. Field of the Invention
The present invention relates to a rectangular conductor for a solar battery, method for fabricating the same and a lead wire for a solar battery using the rectangular conductor, in more particular, to a rectangular conductor for a solar battery, method for fabricating the same and a lead wire for a solar battery using the rectangular conductor, which realizes an excellent electrical characteristics and a less warping of a Si cell of the solar battery when the rectangular conductor is bonded to the Si cell by solder bonding.
2. Description of the Related Art
As shown in FIG. 1, in a solar battery 50 comprising a silicon crystal layer grown on a substrate, a connection lead wire 53 is usually bonded to a predetermined region of a silicon crystal wafer (Si cell) 51, and an electric current flows through the connection lead wire 53.
The connection lead wire 53 comprises a rectangular conductor and a solder plating film formed on a surface of the rectangular conductor, and the solder plating film is used for bonding with the Si cell 51. For example, Japanese Patent Application Laid-Open (Kokai) No. 11-21660 proposes, that a pure copper such as tough pitch copper or oxygen-free copper is used as a rectangular conductor, and a Sn—Pb eutectic crystal solder is used as the solder plating film formed at in outer surface of the rectangular conductor. Further, as disclosed in Japanese Patent Application Laid-Open (Kokai) No. 2002-263880, it is examined in recent years to change a material of a solder plating film into a solder that does not contain Pb (Pb-free solder), because of environmental concerns.
So as to reduce a manufacture cost, a decrease in thickness of a silicon crystal wafer is examined, since a cost of the silicon crystal wafer constitutes the majority of material cost among materials composing a solar battery. However, as shown in FIG. 2, when a silicon crystal wafer is made to be a thin sheet, the Si cell 51 and the connection lead wire 53 bonded with each other by a solder plating 55 might be warped or damaged, due to the temperature variation in heating process during the solder bonding of the connection lead wire 53 or in usage of the solar battery. Accordingly, so as to solve the above problems, needs of a wire rod having a low thermal expansion for the connection lead wire 53 arises.
Japanese Patent Laid-Open (Kokai) No. 2004-204257 proposes a lead wire 70 for a solar battery which has a low thermal expansion, in which an invar (Fe-36 mass % Ni) sheet 73 having a low thermal expansion is clad in a copper material 71, and a solder plating film 75 is formed on a surface of a conductor made of Cu-clad invar as shown in FIG. 3.
TABLE 1 shows material properties of copper-invar-copper (Cu/Fe-36 mass % Ni/Cu) in comparison with material properties of Cu, invar (Fe-36 mass % Ni), and silicon, respectively.
TABLE 1MaterialClad material(Cu/Fe-36 mass% Ni/Cu)Ratio of sheetthickness isFe-362:1:2Cumass % NiSiThermal13.117.01.83.5expansioncoefficient(×10−5/° C.)Young' s modulus125120145170(MPa)Volume21.016.98102.3 × 109resistivity(μΩ · mm)
As understood from TABLE 1, when a rectangular conductor of Cu-clad invar (copper-invar-copper) by using the invar having a low thermal, expansion is manufactured, although a thermal expansion matching with Si can be realized, a volume resistivity increases compared with Cu, so that a power generating efficiency as a solar battery falls due to a decrease of an electric conductivity. Further, since the invar contains about 36% by mass of the nickel, such a rectangular conductor becomes expensive.
In addition, in a three-layer structure lead frame using the copper-invar-copper as a conductor, deformation such as warping may be caused by heterogeneity of orientation or crystal grain of a crystal of copper material disposed on both sides of the invar. These factors cause the deterioration in productivity of solar battery modules or the deterioration in reliability of the generating efficiency after a long-term use. In addition, joint portions of the copper-invar-copper at side surfaces of the lead frame may be exposed to moisture, so that the exposed portion might become like a local battery and finally might be corroded.
Furthermore, in the lead frame using the invar, there is a disadvantage in that a large quantity of wasted materials is produced, since a stamping process is conducted at the time of forming a circuit, thereby increasing a manufacturing cost.